Today's modern hearing aids typically include electronic circuits driving an audio amplifier, which in turn drives a receiver to generate sound into the ear of the user. The electronic circuits and the amplifier are both powered from a battery. Typically, the electronics inside the hearing aid is a combination of digital and analogue circuitry. Such a hearing aid is illustrated in FIG. 1.
With reference to FIG. 1, when the audio amplifier 20 drives the receiver 16 to produce sound, large currents 22 may flow from the battery 24. Because of the impedance 25 of the battery 24, the large currents 22 result in voltage fluctuations on the battery supply line 28 as shown in FIG. 2.
The battery voltage fluctuations appear also on the supply terminals to the hearing aid electronics. Unfortunately the hearing aid electronics will always have a threshold 38 below which the circuitry is not guaranteed to work. This is also shown in FIG. 2. Below the indicated threshold 38 for example analogue circuit cannot be guaranteed to operate as intended, and digital processors/state machines etc. may malfunction or crash at time 40 in FIG. 2.
Because of the limited internal volume for accommodation of electronic components in a hearing aid, typically, there is not sufficient space available for a decoupling capacitor large enough to supply the audio amplifier during output transients.
In WO 2004/034073, a method of current management in a battery powered device is disclosed. The method comprises the steps of comparing the actual supply voltage with a reference, generating a control signal when the supply voltage is below the reference, use the control signal to reduce the load current by disconnection of the load whereby the supply voltage increases, and repeat the previous steps as long as the supply voltage is below the reference voltage. The current limitation is a sampled current limitation. Although the sampling frequency is well above the audible range, frequency convolution may lead to audible deterioration of the output sound due to the time discrete nature of the current limitation.
Thus, there is a need for a hearing aid wherein voltage drops caused by sound amplification is avoided with less deterioration of the sound quality.